This invention relates to a process of melting fine grained, direct reduced iron (DRI), which for at least 80 wt-% has a grain size of not more than 3 mm, in an electric arc furnace containing a bath of liquid iron and on the liquid iron a foamy slag layer, where during the operation of the furnace the DRI is fed through at least one lance from above through the aperture of the lance into the foamy slag layer on the liquid iron bath, said lance being introduced through the roof of said furnace. Among experts, direct reduced iron is also referred to as sponge iron or DRI (direct reduced iron).
Such a process is described in DE 196 08 530 A1, where the DRI is blown onto the iron bath through the lance with a carrier gas consisting mainly of CO2. By that means one prevents the formation of FeO caused by the use of air and the related poor formation of foamy slag, as well as the decrease in quality of steel caused by the injection of nitrogen containing air.
The U.S. Pat. No. 5,433,767 describes the direct reduction of fine grained iron ore in at least two fluidized beds, where hot reduction gas is also used as fluidizing gas. There is produced fine grained DRI which is subsequently liquefied in a melting reactor at temperatures of 1500 to 1700xc2x0 C. and is further reduced. The production of fine grained DRI is also described in the U.S. Pat. No. 5,603,748.
It is the underlying object of the invention to feed the fine gained direct reduced iron also in the hot condition in a simple manner largely free of loss to the iron bath during the operation of the furnace and at the same time forming a low amount of off-gas. In accordance with the invention this is achieved in the above-mentioned process in that the DRI falls through the lance or lances onto the iron bath solely by gravity and without the use of a carrier gas. Apart from the DRI other granular or lumpy iron may also be added to the iron bath, e. g. steel scrap, hot briquetted iron or pig iron. The portion of DRI fed through the lance or lances usually amounts to 85 to 100 weight percent of the total feed material.
During the operation of the furnace gases are constantly ascending from the iron bath which are discharged through the roof of the furnace in form of an off-gas. For the sake of economy it is desirable to minimize the amount of off-gas.
The introduced DRI first of all enters the more or less foamy slag layer, where it is either directly molten or sinks into the iron bath due to its weight and the movement of the liquid bath which is induced by electrical currents. The foamy slag layer prevents that fine grained DRI introduced via the lance is entrained by the ascending gases and discharged from the furnace, which would lead to increased losses of iron. By not using a carrier gas blown through the lance these losses are kept low. Entrained iron may also form accretions in the upper part of the furnace or in the waste gas ducts and thus lead to interruptions in furnace operation.
The electric arc furnace may be operated in the known manner with direct current or alternating current. It is also known to design the electrodes introduced through the furnace roof as vertically movable electrodes and gradually raise the same during the operation of the furnace so that their distance from the bath remains more or less constant during the batch operation.
The fine grained DRI is supplied onto the iron bath through the roof of the furnace through one or several lances which may be provided with water cooling, if necessary. It is expediently prevented that the aperture of the lance or lances gets in contact with the liquid iron or the iron bath. Therefore, each lance is vertically movable, and the aperture of the lance is kept with practically constant distance above the surface of the iron bath. One possibility is that the lance like the electrode is pulled upwards dependent of the rising level of the iron bath. Expediently, the distance of the aperture of each lance from the surface of the iron bath is 3 to 100 cm, and mostly 5 to 50 cm. It is ensured that the lance aperture is always kept inside the foamy slag layer, so that no DRI is entrained to the roof of the furnace by ascending gases.
By separately adding carbon and oxygen it can be ensured in a manner known per se that a stable foamy slag layer is formed on the iron bath and is maintained there during the operation of the furnace. This layer constitutes a reaction zone, which protects the fine grained DRI from reoxidation. At the same time it provides for the immersion of the electrode(s), to protect it or them from oxidation and improve the heat transfer from the arc flare to the melt.
Carbon-containing media and O2-containing gas are supplied to the iron bath through submerged tuyeres. The carbon-containing media may be solid, liquid or gaseous, and as O2-containing gas there is commonly used technically pure oxygen. The submerged tuyeres may be arranged as desired, e.g. in the bottom of the furnace or in the side walls. Expediently, the gas space above the foamy slag comprises one or several injectors to introduce O2-containing gas, so as to effect a partial afterburning of CO.
The iron bath of the furnace usually consists of at least 90 wt-% liquid iron. The furnace may be used for producing hot metal or liquid steel. The liquid metal is withdrawn from the furnace at temperatures in the range from 1300 to 1700xc2x0 C. and preferably at a temperature of at least 1350xc2x0 C. in the case of hot metal and at least 1550xc2x0 C. in the case of steel.